Recent progress in mechanical technology has seen a growing demand for the reduction in size and weight of machines, the enhancement of machine precision, the prolongation of machine life and so forth.
Constant velocity joints are special types of universal couplings which can transmit drive from the final reduction gear to a road wheel axle at constant rotation velocity.
As the constant velocity joints used in automobiles and industrial machines are used at high speeds and under high surface pressure conditions, much better performance is demanded of the grease used to lubricate these joints. This situation will be described in more detail below with reference to constant velocity joints (herein below abbreviated to CVJ) for automobiles.
With the promotion of front wheel drive cars and four wheel drive cars and the like, there has been a marked increase in the use of CVJ in the automobile industry. Cars now have higher output and are smaller and lighter, which imposes severe demands on CVJ durability.
The grease used for CVJ lubrication is also subjected to the demands for better joint durability and lifetime (damage resistance, for example, flaking resistance and seizing resistance).
In response to these demands, sulphur-phosphorus-based extreme-pressure agents comprising sulphurated fat/oil and/or olefin sulphide combined with zinc dithiophosphate, and lithium grease comprising lead-based additives and molybdenum disulphide and the like have mainly been used commercially. In recent years, urea grease, which has excellent heat resistance, has been used more than lithium grease.
Examples of prior art techniques which involve the use of a molybdenum sulphide dialkyldithiocarbamate include Japanese Examined Patent Application No. H4-34590 and Japanese Unexamined Patent Application Nos. H6-57283, H6-330072 and H10-273692.
Japanese Examined Patent Publication No. H4-34590 discloses a system comprising:—(A) molybdenum sulphide dialkyldithiocarbamate; and (B) at least one sulphur-phosphorus-based extreme pressure additive chosen from the group consisting of sulphurated fat/oil, olefin sulphide, tricresyl phosphate, trialkylthiophosphate and zinc dialkyldithiophosphate, compounded into urea grease. However, such systems cannot always be said to be satisfactory under the current severe CVJ working conditions.
Japanese Unexamined Patent Application No. H6-57283 discloses a system comprising molybdenum sulphide dialkyldithiocarbamate, molybdenum disulphide and lead sulphide dialkyldithiocarbamate, compounded into urea grease. However, as this system contains a lead-based additive, it is undesirable in view of increasing concerns over environmental protection.
Japanese Unexamined Patent Application No. H6-330072 discloses adding both (A) molybdenum sulphide dialkyldithiocarbamate and (C) triphenylphosphorothionate to urea grease, but these systems do not simultaneously yield satisfactory damage resistance and abrasion resistance.
T. Sakurai's “Sekiyu Seihin Tenkazai” [Petroleum Product Additives] (p. 262 and thereafter) introduces thiadiazole compounds as lubricant oil additives, and Table 3 on p. 266 suggests that the addition of thiadiazole-based compounds results in excellent sulphuration corrosion prevention with respect to copper and silver. Moreover, Japanese Examined Patent publication No. H4-32880 discloses improved load resistance and extreme pressure properties without corrosion or discoloration of the metal as a result of adding 5,5′-dithiobis(1,3,4-thiadiazole-2-thiol) to lubricating grease, but no mention is made of the problem of balancing damage resistance and abrasion resistance.
Japanese Unexamined Patent Application No. H11-131086 discloses the use of a thiadiazole-based compound as an additive in lubricating grease obtained using a calcium sulphonate complex-based thickening agent, but the thiadiazole-based compound is used here to deactivate metals.
Japanese Unexamined Patent Application No. H10-273692 discloses a grease composition for constant velocity joints comprising diurea as thickener, wherein molybdenum disulphide, phosphorus-free sulphur-based extreme pressure additive and sulphur-nitrogen-based extreme pressure additive are used in combination with (A) molybdenum sulphide dialkyldithiocarbamate. However, it is not clear which specific compounds can be used as the sulphur-nitrogen-based extreme-pressure agent, as “Vanlube 601” (trademark), manufactured by R. T. Vanderbilt, is merely disclosed in the working examples, and said trade name merely confirms that “Vanlube 601” is a heterocyclic sulphur-nitrogen compound. Moreover, according to this technique, the combined use of molybdenum disulphide and phosphorus-free sulphur-based extreme pressure additive is indispensable.
EP-A-0633 304 discloses a urea grease composition comprising a urea grease and, incorporated therein as additives, a sulfurized molybdenum dialkyldithiocarbamate represented by formula (A):-(R1R2N—CS—S)2—Mo2OmSnwherein R1, and R2 each independently represent an alkyl group having from 1 to 24 carbon atoms, m+n=4, m is 0 to 3, and n is 4 to 1, and triphenylphosphorothionate (B).
There is a demand for both satisfactory damage resistance and satisfactory abrasion resistance in the field of lubricating grease compositions for constant velocity joints. There are many greases which have good abrasion resistance but poor damage resistance. There is considerable demand for the discovery of a grease composition for constant velocity joints which provides improvement in both of these physical properties.
It has now been found possible to formulate greases for constant velocity joints containing 5,5′-dithiobis(1,3,4-thiadiazole-2-thiol), having advantageous properties with regard to damage resistance and abrasion resistance.